1. Field of the Invention
The present invention relates to a magneto-optical medium capable of ultrahigh-density recording, which utilizes domain wall displacement upon reproduction.
2. Related Background Art
In recent years, attention has been attracted to magneto-optical disks as rewritable high-density recording media, and there has been a strong demand for further enhancing the recording density of these magneto-optical media to provide them as recording media having a greater capacity. The linear recording density of an optical disk greatly depends on the laser wavelength and the numerical aperture NA of an objective lens of a reproducing optical system, and the detectable range of the spatial frequency upon reproduction of a signal is limited to about NA/.lambda.. Therefore, for actually achieving higher recording density with a conventional optical disk, it is necessary to shorten the laser wavelength .lambda. or enlarge the numerical aperture NA of the objective lens in the reproducing optical system. However, the improvements in the laser wavelength .lambda. and the numerical aperture NA of the objective lens are limited naturally. Therefore, some techniques that the constitution and reading method of a recording medium are devises to improve the recording density have been proposed.
For example, in Japanese Patent Application Laid-Open No. 6-290496, the present applicant previously proposed a magneto-optical medium, a reproducing method and apparatus therefor, by which signals of frequency of less than the optical diffraction limit of an optical system can be reproduced at a high speed without decreasing the amplitude of reproduction signals. When a temperature distribution is formed on a displacement layer of a magneto-optical medium by a heating means such as a light beam, a distribution is formed in the domain wall energy density of the displacement layer as described in this publication. Therefore, a domain wall can be displaced momentarily in a direction low in domain wall energy. As a result, the amplitude of a reproduction signal always becomes fixed and maximum irrespective of the interval (i.e., the length of a record pit) between domain walls of a magnetic domain recorded. Namely, the inevitable reduction in reproduction output attendant on the improvement of linear recording density is greatly improved, so that still higher-density recording becomes feasible.
According to Japanese Patent Application Laid-Open No. 6-290496, the domain wall displacement is a phenomenon occurred in the same track (in a direction of the track), in which it is necessary that a magnetic layer be divided (i.e., magnetically separated among) into tracks. As a method for producing such open magnetic domains (divided domain walls), Japanese Patent Application Laid-Open No. 6-290496 proposes a method of using a rectangularly and deeply grooved substrate or a method of annealing groove portions of a substrate by irradiating them with a laser beam to magnetically modify them. However, the rectangularly and deeply grooved substrate has involved a problem of transferability that the bottom portions of rectangular grooves in a stamper cannot be finely transferred when transferring the pattern of the stamper upon stamping, and so the land portions of the substrate become a U-shaped section, resulting a failure to achieve a sufficient dividing effect. On the other hand, the annealing treatment requires to treat the substrate at a high temperature by irradiation of a laser beam of high power, or the like. The annealing by the laser beam of high power has involved a problem that since the area of portions deteriorated in magnetic property (modified in the magnetic layer) also spread outside tracks depending on the temperature distribution of the laser beam, the area of the magnetic layer, which can be substantially used as an information data division, is narrowed, and so a track density is decreased.